Directional infrared heat lamps are commonly available as BR40 or R-40 lamps having envelopes made from soft glass. Additionally, it is known to make such lamps in PAR38 format from pressed hard glass reflector and lens components. These lamps are often used in agricultural or industrial applications where it is desired that a relatively large flat surface must be uniformly heated. However, presently available heat lamps usually do not perform the desired function well because the lamps have a non-uniform power distribution with maximum radiant intensity on axis dropping to 50% of peak within about 15 degrees of the lamp axis. The radiant beam angle in such cases is about 30 degrees.
The current models of such heat lamps have been based upon the standard lamps designed for general lighting purposes and use most of the same components to keep costs down. The BR40 and R-40 lamps realize some beam spread by use of a frosted inner surface so the maximum bean spread is very limited. The PAR38 lamps can incorporate optical elements in both the reflector and lens and offer much greater control of radiant beam distribution. The available PAR38 general lighting and infrared heat lamps use a reflector that provides only a small amount of beam spread. Most of the spreading is effected by the lens, which is typically formed of a plurality of spherical protrusions or lenticules. For incandescent coil PAR38 lamps with proper design of spherical lenticule radius and layout, a beam spread of nearly 50 degrees can be achieved. Such optics can give a fairly broad flat peak dropping 50% of peak at 25 degrees off-axis. It is not possible to achieve a large area of uniform irradiance on a flat surface using conventional lens optics with spherical lenticules. Futhermore, this type of light distribution is not normally required or desired in general lighting applications.
Even with an isotropic radiating lamp, the irradiance on a flat surface normal to the lamp axis is not uniform and drops substantially with distance from the center because of the inverse square law and the cosine law of illumination. For a point source, the irradiance on a surface is described by E=I/D2·cos β (Equation 1)
Where: I=radiant intensity, D=distance from the source, E=irradiance, β=angle from normal
From this equation it can be shown that for uniform intensity, irradiance fall as cos2 of the angle from normal. For some applications, it is desirable to have a uniform irradiance or a circular flat surface defined by a 50 degree solid angle. A heat lamp of conventional design with the widest possible beam spread, has at least a 60% fall-off in irradiance between center and edge. Most commercially available heat lamps have a much greater variation. This results in a non-uniform temperature distribution across the target area within a 0.6 steradian zone.